I didn't even know what terms to search for to find my answer, so I figured I'd post this newb question in the beginners forum and see what turns up.

So anyway...

I brewed an IPA a few weeks back and for the first time used a 6.5g carboy. Do to my partial boil, I did a 1.5g hop tea that I used as cooling water and then brewed using a 1.5g boil. I believe I spent plenty of time getting the recipe/process down so that it would sit around 55ish IBU, but that is besides the point.

I figured the 1.5g boil probably boiled off .25g or so leaving me with 1.25, plus my 1.5 of cooling hop tea, so I put in 2.25 gallons of top off water. I had no idea how 5g would look in a 6.5g carboy, so I figured I was right on the money at 5g. Anyway, after transferring to secondary in my 5g carboy, it definitely looks to be less than normal (I have used the 5g carboy before). I'm guessing I may have not had 5g in the primary and after transferring I may be closer to 4g.

How in the world can I tell though? I'm worried b/c I do not want to prime with 3/4 cup of corn sugar if I only have 4g in there. Is there a way to measure the height of the beer in my (5g) carboy and convert it to gallons? Even if its a rough guess... ? Or do I simply need to do beer into the bottling bucket first, measure using the lines on the bucket, then add my priming sugar?

I like to put the priming sugar in the bucket first normally and siphon onto it so it circulates into my beer evenly. What if I put the beer in first, then priming sugar? I'm guessing I should stir it or something?

there's no real 'conversion' factor for this sort of thing. Thus your best option is this:

Take your bottling bucket and make sure those lines are accurate by measuring in 5 gallons, one gallon at a time, and verifying that the measures are accurate. Bucket markings are notoriously inaccurate.

Then do as you suggested: move the beer to the bucket, determine how much priming sugar to add based on the verified measurements and proceed.

Then when your carboy is empty measure in a gallon of water. Take some nail polish and put a line on the carboy at the waterline. Repeat until the carboy is full-- I also out half gallon markers on mine. Repeat on the 6.5 gallon carboy. Then your vessels will all be accurately graduated.

Yes, wish I would have marked my carboys before. It never even hit me until now...

I will probably do as you say, but if anyone has a marked 5g carboy already, could you just throw the measurements up (in inches) for to the marks you have on your carboy so I can get a rough estimate in mind.

If it is a 6 1/2 glass carboy then its basically in the shape of a cylindar (the sloped part at the top doesn't count because that is definately more than 5 gal) So to find the Volume of a cylindar we use:

pi*radius^2*height=Volume

Since we know the Volume and are looking for height it becomes

Volume/(pi*radius^2)=height

Now to get everything in the right units we need to look at cubic inches instead of Gallons which, according to Google, is 5 gallons = 1155 cubic inches and pi is~3.14

1155/(3.14*radius^2)=height

So if you find the radius of the carboy (measure from the center to the edge of the bottom) and use the formula above this will give you the approximate height for 5 gallons. I say approximate because this formula will not take into account the slight bevel in the bottom but this should give you a good approximation.